1. Field of the Invention
The present invention relates to a pressure sensor of electric capacitance type (also referred to as the capacitance-type pressure sensor).
2. Description of the Related Art
As a conventional capacitance-type pressure sensor known heretofore, there may firstly be mentioned a device described in Japanese Patent Application Laid-Open Publication No. 233672/1996 (JP-A-8-233672).
According to the conventional technique disclosed in the above publication, a cavity is formed in an auxiliary layer covered with a diaphragm layer by way of passages or gaps formed in the diaphragm layer, whereon these gaps are closed so as not to form any part of a shielding layer deposited on the diaphragm layer in a finishing process. In this way, the finished diaphragm is imparted with a desired deformability.
Further, as a conventional capacitance-type pressure sensor known heretofore, there may secondly be mentioned a device described in Japanese Patent Application Laid-Open Publication No. 307960/1994 (JP-A-6-307960).
In the device disclosed in the publication mentioned just above, pressure sensors are disposed in a matrix-like array, wherein diaphragms formed of polysilicon are disposed above electric conductor patterns formed on a top surface of a silicon substrate in order to determine or detect electric capacitance which change in dependence on compressive distortion. These diaphragms are formed with at least two different sizes, and a number of sensors including respective diaphragms of a same size are accommodated in one subunit. Thus, there are formed at least two subunits, wherein the individual subunits are so implemented that the basic capacitances thereof assume a same value.
The first mentioned conventional technique suffers a problem that the size of the diaphragm which determines the pressure characteristic of the sensor may differ from one to another sensor due to dispersion in the etching condition because of formation of the cavities through partial etching of the auxiliary layer by way of the gaps formed in the diaphragm layer by resorting to a time-based control. Further, in the case where a plurality of sensors are to be interconnected in parallel with one another, there arises necessity for holding the intersensor distance so that the adjacent sensors and cavities are not physically coupled to one another. For this reason, the so-called ineffective region which does not contribute to formation of capacitance to be detected increases, involving increased parasitic capacitance. The increased parasitic capacitance in turn brings about a problem that difficulty is encountered in detecting the pressure with high accuracy.
On the other hand, in the case of the second-mentioned conventional capacitance-type pressure sensor, a large number of sensors to be connected in parallel have independent diaphragms and cavity regions, respectively. Consequently, the so-called ineffective region which does not contribute to formation of capacitance to be detected increases, involving increased parasitic capacitance due to the ineffective region. The increased parasitic capacitance in turn brings about difficulty in detecting the pressure with high accuracy, giving rise to a problem.
In the light of the state of the art described above, it is an object of the present invention to provide a capacitance-type pressure sensor whose the parasitic capacitance is suppressed from increasing and which can ensure pressure detection with enhanced accuracy.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a pressure sensor of electric capacitance type which includes a plurality of pressure sensor units connected in parallel with one another and each formed on a substrate by an electrode, a cavity region and a diaphragm having an electrically conductive film which is disposed in opposition to the aforementioned electrode with the cavity region intervening between the electrode and the diaphragm, wherein diaphragm fixing portions are disposed internally of the cavity region so that a single sheet of the diaphragm is partitionarily and regionally allotted to regions of the plurality of pressure sensor units, respectively.
By virtue of the structure of the capacitance-type pressure sensor described above, ineffective region which does not contribute to the formation of the capacitance to be detected can be minimized with the parasitic capacitance being reduced, whereby the detection accuracy of the sensor can be enhanced significantly.
In a preferred mode for carrying out the invention, the electrode and the diaphragm may be formed on a semiconductor substrate with an insulation film being interposed therebetween.
In another preferred mode for carrying out the invention, etching holes may be formed in the diaphragm for thereby forming the cavity region, which is advantageous in that the throughput of the manufacturing process can be enhanced.
In yet another preferred mode for carrying out the invention, etching holes may be formed at positions on lines interconnecting the diaphragm fixing portions. Owing to this feature, the diaphragm size of the pressure sensor units defined by the diaphragm fixing portions can be protected against differing from one to another sensor unit.
In a further preferred mode for carrying out the invention, a conductive film may be deposited over a surface of a topmost film constituting the diaphragm so as to cover the diaphragm wholly. With this structure, the reliability as well as the detection accuracy of the sensor can be enhanced.
As is apparent from the foregoing, with the structures of the capacitance-type pressure sensor according to the present invention, ineffective region which plays no role in capacitance detection can be minimized, whereby the parasitic capacitance of the sensor can be reduced with the detection accuracy thereof being enhanced significantly.